Simple detection-based LIDAR consists of sending a signal state of light out into an environment that may, or may not, contain a target object, then detecting possible reflected light. Any light reflected to the detector will provide information about the presence of a possible target object. However, when the state of light has a low mean photon number and there is high environmental background noise, accurate inference of the presence of an object is challenging. This problem amounts to discriminating between two states, one containing the reflected signal and noise and the other only noise, so it can be expressed in terms of quantum state discrimination. We determine if an object is present or not by attempting to discriminate the possible states incident on our detector system. The more distinguishable these states are, the more quickly the presence of an object can be recognised or excluded. Quantum illumination exploits non-classically correlated optical modes as the light source to perform object detection[1]}, [2]}, offering a fundamental advantage over classical light sources due to the enhanced distinguishability of non-classical quantum states. Quantum illumination can thus yield improved target discrimination even in noisy quantum channels[3]}, [4]}, however the exact measurement protocol required to achieve the maximal enhancement in sensitivity is currently unknown[5]}. This scheme can be implemented in various ways. If one mode (conventionally “the idler”) is stored locally until a return signal appears at the detector the two modes may be detected in combination to obtain the detection advantage. This is challenging if it requires interference and hence a phase lock between the idler and signal beams, so it is impractical outside a laboratory at optical frequencies. A more practical method entails locally measuring the idler, then using this measurement to condition the signal beam, which is sent to interrogate the target. The hope is that the conditioned signal beam will have enhanced detection probability. Quantum illumination has been shown to have advantages over classical illumination for object detection both experimentally[6]} and theoretically[7]}, when simple detection is used for both the signal and the idler.
